Synthesis of Chromones from 2,3-Allenoic Acids and Benzynes

Significance

A synthesis of chromones from the reaction of 2,3-allenoic acids with Kobayashi benzyne precursors (Y. Himeshima, T. Sonoda, H. Kobayashi Chem. Lett. 1983, 1211; Kobayashi was first to generate benzynes from ortho-TMS arylOTf) is presented. The reaction proceeds smoothly under mild conditions to afford chro­mone products in moderate to good yields; however, the scope of the process was not well investigated. The process is proposed to involve carboxylate addition of the allenoic acid to the generated benzyne to give intermediate A, which instead of forming a coumarin by direct intramolecular Michael addition, undergoes rearrangement to form C via oxetane B opening, followed by intramolecular oxa-Michael addition and hydrolysis to afford the chromone.